1. Field of the Invention
The present invention relates to high-strength heat-resistant steels, and particularly to high-strength heat-resistant steels which are suitable for use in a medium-to-high temperature range up to 540xc2x0 C., and which can be produced at a low cost.
This application is based on Patent Applications Nos. Hei 10-272202 and Hei 11-40618, both filed in Japan, the contents of which are incorporated herein by reference.
2. Description of Related Art
Large portions of materials for pressure-tight parts of piping for use in the highest temperature sections of subcritical-pressure boilers and supercritical-pressure boilers in power plants and waste heat recovery boilers in combined cycle power plants, and semi-high temperature sections of ultra supercritical-pressure boilers are carbon steels and low alloy steels such as 1Cr steel and 2Cr steel.
Specific examples of low alloy steels which have been used are 0.5Mo steel (JIS STBA 12), 1Cr-0.5Mo steel (JIS KA STBA 21, STBA 22, STBA 23) and 2.25Cr-1Mo steel (JIS STBA 24).
Since large portions of the materials for pressure-tight parts of piping are carbon steels and low alloy steels such as 1Cr steel and 2Cr steel, achievement of sufficient strength of the materials for the parts in which they are used, without increasing the use of alloying elements, would largely contribute to reducing the cost for constructing a power plant.
In Japanese Unexamined Patent Application, First Publication (Kokai) No. Hei 10-195593, the present inventors proposed a steel excellent in high-temperature strength as a material suitable for the above uses, comprising C in an amount of 0.01 to 0.1% by weight, Si in an amount of 0.15 to 0.5% by weight, Mn in an amount of 0.4 to 2% by weight, V in an amount of 0.01 to 0.3% by weight, and Nb in an amount of 0.01 to 0.1% by weight, the balance being Fe and unavoidable impurities.
The heat-resistant steel proposed as above is a useful steel, which possesses an enhanced high-temperature strength in comparison with conventional steels although it can be produced at a low cost. However, further enhancement of the high-temperature strength is desired without increasing the cost.
An object of the present invention is to provide a heat-resistant steel which can be produced at a low cost but possesses excellent high-temperature strength, and to provide a production process therefor. Another object of the present invention is to provide a process for producing such a heat-resistant steel possessing excellent high-temperature strength at a low cost by simplified production steps.
In order to achieve the above objects, the following technical measures were taken. That is, the present invention provides a high-strength heat-resistant steel comprising C in an amount of 0.06 to 0.15% by weight, Si in an amount of 1.5% by weight or less, Mn in an amount of 0.5 to 1.5% by weight, V in an amount of 0.05 to 0.3% by weight, and at least one of Nb, Ti, Ta, Hf, and Zr, in an amount of 0.01 to 0.1% by weight, the balance being Fe and unavoidable impurities, wherein the high-strength heat-resistant steel has a structure consisting mainly of a bainite structure.
The high-strength heat-resistant steel according to the present invention, although it contains a small amount of alloying elements, possesses an excellent creep rupture strength, such as 130 MPa extrapolated to 104 hours at 550xc2x0 C., due to a structure consisting mainly of a bainite structure, or preferably consisting of a bainite single-phase structure.
It is preferable that the Si be present in an amount of 0.6% by weight or greater in the high-strength heat-resistant steel according to the present invention if oxidation resistance is regarded as important. The high-strength heat-resistant steel according to the present invention may further comprise at least one of Cr, in an amount of 0.7% by weight or less, and Mo, in an amount of 0.7% by weight or less. The high-strength heat-resistant steel according to the present invention may further comprise B in an amount of 0.005% by weight or less.
The above high-strength heat-resistant steel can be produced by a process for producing a high-strength heat-resistant steel, the process comprising the steps of: normalizing the steel at a temperature in the range of 1100 to 1250xc2x0 C., the steel comprising C in an amount of 0.06 to 0.15% by weight, Si in an amount of 1.5% by weight or less, Mn in an amount of 0.5 to 1.5% by weight, V in an amount of 0.05 to 0.3% by weight, and at least one of Nb, Ti, Ta, Hf, and Zr, in an amount of 0.01 to 0.1% by weight, the balance being Fe and unavoidable impurities; hot-working the steel at a final reduction ratio of 50% or greater at a temperature within the range in which austenite recrystallizes, so as to produce a hot-worked product; and cooling the hot-worked product to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed.
Alternatively, the above high-strength heat-resistant steel can be produced by a process comprising the steps of: preparing an ingot having the above composition; hot-working the ingot, during the process of cooling the ingot, at a final reduction ratio of 50% or greater at a temperature within the range in which austenite recrystallizes, so as to produce a hot-worked product; and cooling the hot-worked product to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed.
In the above processes of the present invention, after the step of hot-working at a temperature within the range in which austenite recrystallizes, the hot-worked product may be additionally hot-worked at a temperature in the range of 950xc2x0 C. to the Ar3 point, and then the step of cooling the hot-worked product to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed may be conducted. Moreover, after the step of cooling to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed to produce a cooled product, the step of tempering the cooled product at the A1 point or a lower temperature may be conducted.
When a high-strength heat-resistant pipe is produced according to the present invention, the process may comprise the steps of: normalizing a steel having the above composition at a temperature in the range of 1100 to 1250xc2x0 C.; piercing the steel to produce a pierced product; and cooling the pierced product to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed. Alternatively, the process may comprise the steps of: preparing an ingot having the above composition; piercing the ingot, during the process of cooling the ingot, at a temperature within the range in which austenite recrystallizes, so as to produce a pierced product; and cooling the pierced product to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed.
The effects of the present invention are explained in the following.
The heat-resistant steel according to the present invention, although it is a low alloy, possesses a creep rupture strength superior to those of conventional heat-resistant steels, due to its specific chemical composition and a structure consisting mainly of a bainite structure. Accordingly, this effect can be made more remarkable by making the structure a single-phase structure. In the present invention, a prescribed amount of at least one of Cr and Mo, which may be incorporated, improves the hardenability, and contributes to the formation of the single-phase bainite structure. In addition, B improves the hardenability by restricting the generation of ferrite, and contributes to the formation of the single-phase bainite structure.
The production process of the present invention, according to which a steel of a specific composition is normalized at a temperature in the range of 1100 to 1250xc2x0 C., then hot-worked at a final reduction ratio of 50% or higher at a temperature within the range in which austenite recrystallizes, and then cooled to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed, allows production of a high-strength heat-resistant steel, having a structure consisting mainly of a bainite structure, which, although it is a low alloy, possesses a creep rupture strength superior to those of conventional heat-resistant steels.
The other production process, according to which an ingot of specific composition is prepared, then hot-worked, during the process of cooling the ingot, at a final reduction ratio of 50% or greater at a temperature within the range in which austenite recrystallizes, and then cooled to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed, allows production of a high-strength heat-resistant steel which possesses a creep rupture strength superior to those of conventional heat-resistant steels at a low cost in a simplified production process.
In the case in which a pipe such as a boiler tube is manufactured, piercing may take place at a temperature within the range in which austenite recrystallizes, and then cooling to room temperature or to a temperature lower than the temperature at which the transformation to bainite is completed. This production process allows production of a high-strength heat-resistant pipe which, although it is a low alloy, possesses a creep rupture strength superior to those of conventional heat-resistant pipes.